Modern Machine-Shop Practice Part 28

A defect in taps which it has been found so far impracticable to eliminate is the alteration of pitch which takes place during the hardening process. The direction as well as the amount of this variation is variable even with the most uniform grades of steel, and under the most careful manipulation. Mr. John J. Grant, in reply to a communication upon this subject, informs me that, using Jones and Colver's (Sheffield) steel, which is very uniform in grade, he finds that of one hundred taps, about 5 per cent. will increase in length, the pitch of the thread becoming coa.r.s.er; 15 per cent. will suffer no appreciable alteration of pitch, and 80 per cent. will shrink in length, the pitch becoming finer, and these last not alike. But it must be borne in mind that with different steel the results will be different, and the greater the variation in the grade of the steel the greater the difference in the alteration of pitch due to hardening.

It is further to be observed that the expansion or contraction of the steel is not constant throughout the same tap; thus the pitches of three or four consecutive teeth may measure correct to pitch, while the next three or four may be of too coa.r.s.e or too fine a pitch.

There is no general rule, even using the same grade of steel, for the direction in which the size of a tap may alter in hardening, as is attested by the following answers made by Mr. J. J. Grant to the respective questions:--

"Do the taps that shorten most in length increase the most in diameter?"

Answer.--"Not always; sometimes a tap that shortens by hardening becomes also smaller in diameter, while sometimes a tap will increase in length, and also in diameter from hardening."

"Do taps that remain of true pitch after hardening remain true, or increase or diminish in diameter?"

Answer.--"They will generally be of larger diameter."

"Do small taps alter more in diameter from hardening than large ones?"

Answer.--"No; the proportion is about the same, and is about .002 per inch of diameter."

"What increase in diameter do you allow for shrinkage in hardening of hob taps for tapping solid dies?"

Answer.--"As follows:--

Diameter of Shrinkage Hob Tap about

1/4 inch .003 1/2 " .003 3/4 " .005 1 " .008"

"Suppose a tap that had been hardened and tempered to a straw color contained an error 1/1000 inch both in diameter and in pitch, was softened again, would it when soft retain the errors, or in what way would softening affect the tap?"

Answer.--"We have repeatedly tried annealing or softening taps that were of long or short pitch caused by tempering, and invariably found them about the same as before the annealing. The second tempering will generally shorten them more than the first. Sometimes, however, a second tempering will bring a long pitch nearer correct."

"Do you soften your taps after roughing them out in the lathe?"

Answer.--"Never, if we can possibly avoid it. Sometimes it is necessary because of improper annealing at first. The more times steel is annealed the worse the results obtained in making the tool, and the less durable the tool."

The following are answers to similar questions addressed to the Morse Twist Drill and Machine Co.:--

"The expansion of taps during hardening varies with the diameter. A 1-inch tap would expand in diameter from 1/1000 to 3/1000 inch."

"Taps above 1/2 inch diameter expand in diameter to stop the gauge every time."

"The great majority of taps contract in pitch during the hardening, they seldom expand in length."

"The shortening of the pitch and the expansion in diameter have not much connection necessarily, though steel that did not alter in one direction would be more likely to remain correct in the other."

"There does not seem to be any change in the diameter or pitch of taps if measured after hardening (and before tempering) and again after tempering them."

"Taps once out in length seem to get worse at every heating, whether to anneal or to harden."

[Ill.u.s.tration: Fig. 355.]

It will now be obvious to the reader that the diameter of a tap, to give a standard sized bolt a required tightness of fit, will, as a general rule, require to vary according to the depth of hole to be tapped, because the greater that depth the greater the error in the pitch.

Suppose a tap, for example, to get of finer pitch to the amount of .002 per inch of length, then a hole an inch deep and tapped with that tap would err .002 in its depth, while a hole two inches deep would err twice as much in its depth.

[Ill.u.s.tration: Fig. 356.]

Therefore a bolt that would be a hand fit (that is, screw in under hand pressure) in the hole an inch deep would require more force, and probably the use of a wrench, to wind it through the hole 2 inches deep; hence in cases where a definite degree of fit is essential, the reduction in diameter of the male screw or thread necessary to compensate for the error in the tap pitch must vary according to the depth of the hole, and the degree of error in the tap.

[Ill.u.s.tration: Fig. 357.]

It is obvious that the longer a tap is the greater the error induced by hardening, and it often becomes a consideration how to tap a long hole, and obtain a thread true to pitch. This may be accomplished as follows.

Several taps are made of slightly different diameters, the largest being of the required finished size. Each tap is made taper for a distance of two or three threads only, and is hardened at this tapered end, but left soft for the remainder of its length. The smallest tap is used first, and when it has tapped a certain distance, a larger one is inserted, and by continuing this interchange of taps and slightly varying the length of the taper, the work may be satisfactorily done.

To test the accuracy, or rather the uniformity, of a thread that has been hardened, a sheet metal gauge, such as at G or at G' (Fig. 355), may be used, there being at _a_ and _b_ teeth to fit the threads. If the edge of the gauge meets the tops of the threads, then their depth is correct. If it is desired to test only the pitch, then the gauge may be made as at G', where, as is shown in the figure, the edge of the gauge clears the tops of the threads, and in this way may be tried at various points along the thread length.

[Ill.u.s.tration: Fig. 358.]

A method of truing hardened threads proposed by the author of this work in 1877, and since employed by the Pratt and Whitney Company to true their hardened steel plug-thread gauges, is as follows:--A soft steel wheel about 3-1/2 inches in diameter, whose circ.u.mference is turned off to the shape of the thread, is mounted upon the slide rest of a lathe, and driven by a separate belt after the manner of driving emery wheels; this wheel is charged with diamond dust, which is pressed into its surface by a roller, hence it grinds the thread true.

The amount allowed for grinding is 3/1000 inch measured in the angles of the thread, as was shown in Figs. 280 and 281.

In charging the wheel with diamond dust it is necessary to use a roller shaped as in Fig. 356, so that the axis of the roller R and wheel W shall be at a right angle, as denoted by the dotted lines. If the roller is not made to the correct cone its action will be partly a rolling and partly a sliding one, and it will strip the diamond dust from the wheel rather than force it in, the reasons for this being shown in Figs. 57 and 58 upon the subject of bevel-wheels.

[Ill.u.s.tration: Fig. 359.]

Taps for lead and similar soft metal are sometimes made with three flat sides instead of grooves. The tapping holes may in this case be made of larger diameter than the diameter of the end of the tap thread, because the metal in the hole will compress into the tap thread, and so form a full thread. Taps for other metal have also been made of half-round section. Fig. 357 represents a tap of oval cross section, having two flutes, as shown, but it may be observed that neither half-round nor oval taps possess any points of advantage over the ordinary forms of three or four fluted taps, while the former are more troublesome and costly to manufacture.

[Ill.u.s.tration: Fig. 360.]

When it is required to tap a hole very straight and true, it is sometimes the practice to provide a parallel stem to the tap, as shown in figure at C. This stem is made a neat working fit to the tapping hole, so that the latter serves as a guide to the tap, causing it to enter and to operate truly.

TAP WRENCH.--Wrenches for rotating a tap are divided into two princ.i.p.al cla.s.ses, single and double wrenches. The former has the hole which receives the squared end of the tap in the middle of its length, as shown in Fig. 358 at E, there being a handle on each side to turn it by.

[Ill.u.s.tration: Fig. 361.]

The single wrench has its hole at one end, as shown in Fig. 359 at D, and is employed for tapping holes in locations where the double wrench could not be got in.

[Ill.u.s.tration: Fig. 362.]

[Ill.u.s.tration: Fig. 363.]

In some cases double tap wrenches are made with two or three sizes of square holes to serve as many different sizes of taps, but this is objectionable, because unless the handles of the wrench extend equally on each side of the tap, the overhanging weight on one side of the tap exerts an influence to pull the tap over to one side and tap the hole out of straight. For taps that have square heads the wrench should be a close but an easy fit to the tap head, otherwise the square corners of the tap become rounded. For the smaller sizes of taps, adjustable wrenches, such as shown in Fig. 360, are sometimes employed. These contain two dies; the upper one, which meets the threaded end of C, being a sliding fit, and the joint faces being formed as shown at A, B.

By rotating the handle C its end leaves the upper die, which may be opened out, leaving the square hole between the dies large enough to admit the squared tap end. After the wrench is placed on the tap, C is rotated so as to close the dies upon the tap.

[Ill.u.s.tration: Fig. 364.]

[Ill.u.s.tration: Fig. 365.]

When the location of the tapping hole leaves room for the wrench to rotate a full circle, C is screwed up so that the dies firmly grip the tap head, which preserves the tap head; but when the wrench can only be rotated a part of a revolution, C is adjusted to leave the dies an easy fit to the tap head, so as to enable the wrench to be removed from the tap head with facility and again placed upon the tap head. C is operated by a round lever or pin introduced in a hole in the collar, or the collar may be squared to receive a wrench.